1. Field of the Invention
This invention relates generally to optical couplers and more particularly, to a novel single mode coupler which is temperature insensitive, hermetically sealed and which exhibits virtually zero loss. The coupler of this invention is produced by means of optical contact of ground and polished fiber components.
2. Description of the Prior Art
Optical couplers are well known and various methods have been proposed to manufacture and assemble single mode optical couplers. The present invention is primarily directed to single mode, evanescent wave, fiber optic couplers.
The function of a fiber optic coupler is to divide or combine the optical power between two fibers. In the particular case of single-mode fibers, this can be achieved through the interaction of the evanescent wave fields in the two fibers, which extend outside the core.
Fiber optic couplers are used in two major fields; namely, optical sensors and communication. The performance requirements are similar in both cases and these include (a) low loss, (b) specified coupling ratio from 0 to 1, (c) minimum temperature dependence of (a) and (b), and (d) mechanical stability.
For field-useable optical sensors, temperature stability is very important. In the case of ring-resonator sensors, very low loss is also critical, as is the ability to tune the coupling ratio very accurately. Both these factors affect the resonator finesse and hence the sensor sensitivity.
For communications systems such as local area networks or data bus lines, the lower the loss, the more couplers (branches) can be used. The tolerable loss is well under 1% in many cases. Both temperature and mechanical stability and the ability to set the coupling ratio precisely are fundamental considerations for the system designer.
The optical couplers found in the prior art suffer from one or more important drawbacks, including sensitivity to temperature, relatively large loss and mechanical instability.
Several of these couplers and the methods of producing them are described in the literature and in prior issued patents. Reference is made in particular to U.S. Pat. No. 4,387,954 which issued on June 14, 1983 to Beasley and assigned to Gould Inc.; U.S. Pat. No. 4,307,933 issued to Palmer et al and assigned to General Dynamics; U.S. Pat. No. 4,386,822 issued to Bergh and assigned to The Leland Stanford Junior University and U.S. Pat. No. 4,410,275 granted to Shaw et al.
The disadvantages of these prior art methods are well known as stated hereinbefore, and it is to overcoming these problems to which the present invention is directed.
More specifically, with "twist-etched couplers", such as those disclosed in S. K. Sheem and T. G. Giallorenzi, (1979) Optics Letters, 4, 29 and in P. C. Tran et al, (1981) IEEE, J. Quantum Electronics QE-17, 988, the etching removes the fiber cladding from all around and weakens the fiber. This coupler has not been demonstrated with low loss and with temperature stability, because of the difficulty in finding a suitable potting material. A further disadvantage is that the coupling ratio must be fixed during fabrication.
Fused biconical couplers, such as those disclosed in B. S. Kawasaki et al, (1981) Optics Letters 6, 327 and T. Bricheno and A. Fielding, (1984) Electronics Letters 20, 230, have not been demonstrated with very low losses, due to the deformation of the fiber during fusion. The coupling ratio is fixed during fabrication as with the twist-etched couplers.
C. A. Villarruel and R. P. Moeller, in the (1981) Electronics Letters disclose a combination of the twist-etched couplers and the fused bioconical couplers but such combination suffers from the same drawbacks as each of the foregoing types.
The special D-shaped fiber couplers, as disclosed by L. G. Schoner et al, (1982) Electronics Letters 18, 567 is a coupler formed by fusing two D-shaped fibers thus obviating the need for grinding and polishing. However, large losses were reported as well as substantial polarization dependence of coupling ratio due to the distorted fiber shape.
Polished block couplers such as those disclosed in R. A. Bergh et al, (1980) Electronics Letters 16, 261; M. J. F. Digonnet and H. J. Shaw, (1982) IEEE, J. Quantum Electronics, QE-18,746; O. Parriaux et al, (1981) Applied Optics 20, 2420 and P. Jaccard et al, (1983) 9th European Conference on Optical Communications, H. Melchior and A. Sollberger (eds), p. 409, use index matching oils which makes them very sensitive to temperature, as the refractive index of the oil changes rapidly with temperature. The oil must also be of high purity such that light is not scattered by particulate matter in the oil interface layer.
Typically, as stated in Digonnet et al (supra), there is a 1:2 change in coupling ratio over 25.degree. C. to 30.degree. C. Both the coupling ratio and loss are drastically affected by temperature.
Polarization preserving and polarizing fiber can also be used to make a polarization preserving (PP) coupler which is very important for sensors where polarization maintenance is critical. However, even PP couplers as disclosed in the prior art (see M. Kawachi et al, (1982) Electronics Letters 18, 962; C. A. Villarruel et al, (1983) Electronics Letters 19, 18; R. B. Dyott and J. Bello, (1983) Electronics Letters 14, 601; and B. K. Nayar and D. R. Smith, (1983) Optics Letters 8, 453) suffer generally from the drawbacks as stated hereinbefore.